It was a summer night when Lena, Tom, Igor and Hania walked through the gates of the old castle on the hill above the town for the first time. The building had been standing empty for years, and local residents told all sorts of unbelievable stories about it. The 14-year-olds had organised a sleepover in tents near the walls, under the pretext of a summer camp; in reality, however, they were hoping for something more than a simple bonfire and sausages. Already on the first evening, something started to happen. As the sun slowly set and the mist drifted across the valley, Lena noticed a flash of light in one of the castle windows. - Did you see that! - she called out, pointing her finger at the north tower. The rest watched in silence for a moment until Tom laughed nervously: - 'Probably just reflected the light of the car from the road.... However, after a moment Igor took out his binoculars and pointed them towards the castle. - 'No... It's something else,' he muttered. - The light is moving from inside.... Everyone's eyes widened with excitement. They couldn't pass up such an opportunity. As if on cue, they all started packing the most necessary things: torches, phone, notebook and Hania's poorly sharpened pocket knife. With difficulty, they squeezed through the thickets growing over the castle's moat and were already standing under the high stone stairs leading to the courtyard. The castle seemed even bigger when they looked at it from below. Every whisper echoed through the dark, empty corridors. Lena tried to summon up her courage and joked: - 'All right, who goes in first? - Me! - chuckled Tom, although his laughter sounded a little too loud. They climbed the stairs until the big wrought iron gate slammed behind them. They stopped, listening to the silence broken only by the distant hooting of an owl. A deafening clatter echoed underfoot as they walked along the slabs of the courtyard. An image of light moved across the tower wall. Lena raised her torch and the beam illuminated something on the wall. It was a strange mark, carved into the stone: it resembled a spiral surrounded by symbols they had never seen before. They stopped, looking at each other uncertainly. As Igor came closer, suddenly something rustled among the bushes on the other side of the courtyard. There was a flash of light again, this time clearer - as if someone had run through a window recess. Something fell from Tom's hand, and Lena heard her accelerated breathing. - Did you hear that? - asked Hania in a whisper. Suddenly the door to the tower shook violently, and a thick grey mist poured out of its gap. Someone - or something - began to approach up the stairs towards them. The four teenagers stood motionless, with their hearts pounding, waiting to see what would emerge from the misty corridor....